Remove shproj

[s2quake]

I have used shproj files to share sources across multiple csproj files.
While it is possible to add a csproj using the dotnet sln command, there is no way to add an shproj file. the only way is to use Solution Explorer in Visual Studio in a Windows environment.
Crossing operating systems to manage shproj files is quite inefficient.
Fortunately, I found a way to share sources without shproj and just using the msbuild property.